Pixel driver circuit with load-balance in current mirror circuit

ABSTRACT

A pixel circuit for use in a display comprising a plurality of pixels is provided. The load-balanced current mirror pixel circuit can compensate for device degradation and/or mismatch, and changing environmental factors like temperature and mechanical strain. The pixel circuit comprises a pixel drive circuit comprising, switching circuitry, a current mirror having a reference transistor and a drive transistor, the reference transistor and the drive transistor each having a first and second node and a gate, the gate of the reference transistor being connected to the gate of the drive transistor; and a capacitor connected between the gate of the reference transistor and a ground potential, and a load connected between the current mirror and a ground potential, the load having a first load element and a second load element, the first load element being connected to the first node of the reference transistor and the second load element being connected to the first node of the drive transistor.

FIELD OF INVENTION

The present invention relates to circuitry for use in an active matrixdisplay, and more particularly to a current drive circuitry used todrive the electro-luminescent elements.

BACKGROUND OF THE INVENTION

OLED based displays have gained significant interest recently for manydisplay applications because of their faster response times, largerviewing angles, higher contrast, lighter weight, lower power, andamenability to flexible substrates, as compared to liquid crystaldisplays (LCDs).

The simplest way of addressing an OLED display is to use a passivematrix format. Although passive matrix addressed OLED displays arealready in the marketplace, they do not support the resolution neededfor next generation displays, which use high information content (HIC)formats. HIC formats are only possible with an active matrix addressingscheme.

Active matrix addressing involves a layer of backplane electronics,based on thin-film transistors (TFTs). These thin film transistorsprovide the bias voltage and drive current needed in each OLED pixel andmay be fabricated using amorphous silicon (a-Si:H), polycrystallinesilicon (poly-Si), organic, polymer, or other transistor technologies.When compared to passive matrix addressing, active matrix addressinguses a lower voltage on each pixel and the current throughout the entireframe period is a low constant value. Thus, active matrix addressingavoids the excessive peak driving and leakage currents associated withpassive matrix addressing. This increases the lifetime of the OLED.

LCDs are electric field driven devices. OLEDs, on the other hand, arecurrent driven devices. Thus, the brightness and stability of the lightemitted by a given OLED used in a display is dependent on the operationof the TFTs in the current drive circuit. Thus AMOLED displays are farmore sensitive to TFT instabilities including, spatial and temporalvariations in transistor threshold voltage, mobility instability, andmismatch issues. These instabilities need to be addressed for widespreaduse of OLED based displays.

FIG. 1 presents a graph of threshold voltage 'shift vs. stress voltagefor various times for amorphous silicon based TFTs. It is readilyapparent from FIG. 1 that the threshold voltage of the transistorsvaries over time. If these transistors were used in a display, thevariation in threshold voltage would likely result in variation in thebrightness of the OLED across the array and/or a decrease in brightnessover time, both of which are unacceptable.

A simple pixel driver circuit is shown in FIG. 2. This “2T” circuit is avoltage programmed circuit. Such a circuit is not practical for OLEDdisplays as such a circuit can not compensate for variations intransistor threshold voltage. One solution to this variation inthreshold voltage is to use a current programmed circuit to drive theOLED of the pixels. Current programming is a good method for drivingAMOLED displays since the OLED is a current driven device, and itsbrightness is approximately linearly dependent upon the current flowingthrough it.

One such current programmed circuit is presented in FIG. 3. This circuitincorporates a current-mirror which compensates for any shift ormismatch in the threshold voltage of the drive transistor T4 whichensures that the brightness of the OLED does not decrease over time.This feature of the circuit allows its drive characteristics to be muchimproved as compared to the 2T circuit of FIG. 2.

When programming the circuit of FIG. 3, V_(ADDRESS) is high and acurrent I_(DATA) is applied. This current initially flows throughtransistor T1 and charges capacitor C_(S). As the capacitor voltagerises, T3 begins to turn on and I_(DATA) starts to flow through T2 andT3 to ground. The capacitor voltage stabilizes at the point when all ofI_(DATA) flows through T2 and T3, and none through T1. This process isindependent of the threshold voltage V_(T) of transistors T3 and T4.

The gates of T3 and T4 are connected, so the current flowing through T3is mirrored in T4. This topology allows us to have on-pixel current gainor attenuation depending on the sizing of T3 and T4, so that therespective data current can be proportionately smaller or larger thanthe OLED current. In an active matrix array, pixels are scanned andprogrammed in a row-by-row fashion. The time taken to scan all rows (oneframe) is called the frame time. During array operation, the switchingTFTs (T1 and T2) are ON only once in the frame time.

However, existing current programmed circuits do not adequately addresslong-term stability in the OLED drive current due to differentialVt-shift and other bias, temperature, or mechanical stress relateddegradations and mismatches in the current mirror.

SUMMARY OF THE INVENTION

The present invention relates to a circuit for driving light emittingelements in a display and more particularly relates to a current drivecircuit that implements a current mirror wherein each transistor of thecurrent mirror is connected to a load.

It is an object of the invention to provide improved AMOLED DisplayBackplanes and Pixel Driver Circuits.

Accordingly, it is an object of the present invention to provide pixelcurrent driver circuits for active matrix organic light emittingdisplays (AMOLED), capable of providing stable and predictable drivecurrents, in the presence of device degradation and/or mismatch, andchanging environmental factors like temperature and mechanical strain.The latter is particularly important for mechanically flexible AMOLEDdisplays.

According to an aspect of the invention a pixel circuit for use in adisplay comprising a plurality of pixels is provided. The pixel circuitcomprises a pixel drive circuit comprising, switching circuitry, acurrent mirror having a reference transistor and a drive transistor, thereference transistor and the drive transistor each having a first andsecond node and a gate, the gate of the reference transistor beingconnected to the gate of the drive transistor; and a capacitor connectedbetween the gate of the reference transistor and a ground potential, anda load connected between the current mirror and a ground potential, theload having a first load element and a second load element, the firstload element being connected to the first node of the referencetransistor and the second load element being connected to the first nodeof the drive transistor.

According to another aspect of the invention a pixel circuit for use ina display comprising a plurality of pixels is provided. The pixelcircuit comprises a pixel drive s circuit comprising, switchingcircuitry, a current mirror having a reference transistor and a drivetransistor, the reference transistor and the drive transistor eachhaving a first and second node and a gate, the gate of the referencetransistor being connected to the gate of the drive transistor, thesecond node of the reference and drive transistors connected to a groundpotential, and a capacitor connected between the gate of the referencetransistor and a ground potential, and a load connected between thecurrent mirror and a potential.

This summary of the invention does not necessarily describe all featuresof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1 shows a graph of threshold voltage shift v. gate stress voltagefor various times for thin film transistors made from amorphous silicon;

FIG. 2 shows a schematic diagram of a 2T voltage-programmed pixel drivercircuit;

FIG. 3 shows a schematic diagram of a 4T current-programmed drivercircuit;

FIG. 4 shows a block diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 5A shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 5B shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 5C shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 6A shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 6B shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 6C shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 7A shows a block diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 7B shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 7C shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention;

FIG. 7D shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention; and

FIG. 7E shows a schematic diagram of a current-programmed driver circuitaccording to an embodiment of the invention.

The above objects and features of the present invention will become moreapparent by the following description of the preferred embodiments withreference to the attached drawings.

DETAILED DESCRIPTION

It has been found that the long-term stability of the OLED drive currentcan be addressed by providing a load to each transistor of the currentmirror of a current based drive circuit.

A block diagram of a pixel driver circuit according to one aspect of theinvention is shown in FIG. 4. The driver circuit can generally beconsidered to include a switching circuit 22, a current mirror 24 and aload 26. Of particular note is that the load 26 is configured, withrespect to the current mirror 24, such that the two transistors of thecurrent mirror 24 have a load connected to them. In the configurationshown in FIG. 4 the load 26 is connected between the current mirror 24and ground with connections 28 and 30. Where the connections 28 and 30are each connected to a node of a transistor of the current mirror andthe load 26. This architecture provides for a balancing of the loadbetween the transistors of the current mirror. Embodiments of theinvention that implement this architecture will now be presented.

In the embodiment presented in FIG. 4 the switching circuit 22 isconnected to two select lines, namely V-sel1 and V-sel2. The embodimentspresented in FIGS. 5A-5C, 6A-6C and 7A-7E likewise have two selectlines. The switching circuit 22 is further connected to a single dataline, I-data.

The circuits presented in FIGS. 5A to 5C have the same basicarchitecture as the circuit presented in FIG. 4, i.e. both transistorsof the current mirror are connected to the load 26. The circuits ofFIGS. 5A to 5C present type and configuration variations for the load26.

In FIG. 5A the current mirror 24 includes a reference transistor 31, adrive transistor 33. The transistors 31 and 33 are thin film transistorswhich have an amorphous silicon channel. A storage capacitor 25 isincluded in the current mirror 24. The gates of the transistor 31 andthe transistor 33 are tied together and both connected to a plate of thestorage capacitor 25. The other plate of the storage capacitor Cs isconnected to ground. The source of the reference transistor 31 isconnected to potential Vc and the drain is connected to the switchingcircuit 22. Connecting the source to the potential Vc allows the twosides of the current mirror to be balanced with proper biasing. Thesource of the drive transistor 33 is connected to a light emitting diode32 and the drain is connected to V_(DD). In this embodiment the lightemitting diode 32 is an organic light emitting diode (OLED).

FIG. 5B is a schematic diagram of a pixel driver circuit according toanother embodiment of the invention. In this embodiment the source ofthe reference transistor 31 and the drive transistor 33 are connected tolight emitting diodes 36 and 32, respectively.

FIG. 5C presents the currently preferred configuration for the load 26.The transistors 31 and 33 are tied together using a connection 37. InFIG. 5C the connection 37 is pictorially located within the load 26. Thecurrent embodiment is not limited by this representation. A single OLED37 is connected to the common connection 37.

FIGS. 6A to 6C present embodiments of the invention wherein the currentmirror 24 and the load 26 are the same as the embodiment presented inFIG. 5C while various configurations of the switching circuitry areprovided. The switching circuits presented in FIGS. 6A to 6C each have afeedback transistor 44 and a switch transistor 46.

In the circuit presented in FIG. 6A one terminal of the feedbacktransistor 44 and one terminal of the switch transistor 46 are connectedto data line I-data. The second terminal of the feedback transistor 44is connected to the drain of reference transistor 31 while the secondterminal of the switch transistor 46 is connected to the gate of thereference and drive transistors 31 and 33, respectively. Finally, thegate of the feedback transistor 44 and switch transistor 46 is connectedto the select line V-sel1 and select line V-sel2, respectively.

In the embodiment presented in FIG. 6B the first terminal of the switchtransistor 46 is connected to the data line I-data while the firstterminal of the feedback transistor 44 is connected to the secondterminal of the switch transistor 46 which is connected to the gate ofthe reference and drive transistors 31 and 33, respectively. The secondterminal of the feedback transistor 44 is connected to the drain of thereference transistor 31. Finally, the gate of the feedback transistor 44and switch transistor 46 is connected to the select line V-sel2 andselect line V-sel1, respectively.

In the embodiment presented in FIG. 6C the first terminal of the switchtransistor 46 is connected to the data line I-data while the firstterminal of the feedback transistor 44 is connected to the secondterminal of the switch transistor 46 which is connected to the drain ofthe reference transistor 31. The second terminal of the feedbacktransistor 44 is connected to the gate of the reference and drivetransistors 31 and 33, respectively. Finally, the gate of the switchtransistor 46 and feedback transistor 44 is connected to the select lineV-sel1 and select line V-sel2, respectively.

The circuits that have been considered are embodiments of the circuitpresented as a block diagram in FIG. 4. An alternative embodiment of thecircuit architecture of FIG. 4 is presented in FIG. 7A. The organizationof the switching circuit 22 and the current mirror 24 is the same as theembodiment presented in FIG. 4. In this embodiment the load 26 isarranged such that it is between the potential V_(DD) and the currentmirror 24. FIG. 7B-7E present embodiments of the invention based on theblock diagram of FIG. 7A. These embodiments implement the same circuitfor the current mirror 24 while the configuration of, the load 26varies.

In the embodiment presented in FIG. 7B the load 26 includes lightemitting diodes 40 and 42. The diodes 40 and 42 are connected betweenthe potential V_(DD) and the drain of reference transistor 31 and drivetransistor 33, respectively. The sources of the reference transistor 31and the drive transistor 33 are connected to ground. The gates of thereference transistor 31 and the drive transistor 33 are tied togetherand connected to both the switching circuit 22 and a plate of thestorage capacitor 25. In the embodiment presented in FIG. 7C the lightemitting diode 40 is connected to a potential V_(c) and the diode 42 isconnected to the potential V_(DD). The embodiments presented in FIGS. 7Dand 7E differ from the embodiments of FIGS. 7B and 7C, respectively, inthat the light emitting diode 40 is replaced with a transistor 47. Thegate of transistor 47 is connected to a third select line V-sel3, afirst terminal is connected to a potential and a second terminal isconnected to the source terminal of reference transistor 31.

In the schematic diagram of FIGS. 5B, 7B, and 7C there are two OLEDs ineach pixel. Such a double OLED structure is formed by partitioning thebottom electrode of the OLED of each pixel into two electrodes.Partitioning of the electrode provide for the formation of two OLEDs ineach pixel. One of the OLEDs is connected to the drive transistor andthe other is connected to the reference transistor. Therefore the loadof reference and drive transistors is the same, resulting in aminimization of mismatches between these two transistors. It is notedthat the ratio between the areas of the two OLEDs and the gain of thecurrent mirror can be engineered to achieve desired circuit performance.

According to an alternative embodiment of the invention the transistorscan be any appropriate material for the fabrication of thin filmtransistors including polycrystalline silicon, polymer and organicmaterials. In particular this embodiment considers appropriate changesfor including p-type TFTs that are relevant to persons skilled in theart.

According to another alternative embodiment of the invention the pixeldrive circuits do not include the capacitor Cs.

According to another alternative embodiment of the invention theswitching circuit 22 is appropriate for the use with a single selectline.

According to another alternative embodiment of the invention thetransistors of the pixel driver circuits may have more than one gate. Inparticular the transistors may be dual gate transistors.

According to another alternative embodiment of the invention there ismore than one driver circuit for a given pixel. In particular there maybe three pixel driver circuits as would be appropriate for pixels in anRGB or colour display.

The present invention has been described with regard to one or moreembodiments. However, it will be apparent to persons skilled in the artthat a number of variations and modifications can be made withoutdeparting from the scope of the invention as defined in the claims.

What is claimed is:
 1. A pixel drive circuit for use in a display, thepixel drive circuit comprising: switching circuitry for providing areference current input; a current mirror for providing a drive currentto a light emitting device based on the reference current, the currentmirror including: a drive thin film transistor for conveying the drivecurrent through the light emitting device, the drive thin filmtransistor being connected in series with the light emitting device; areference thin film transistor for receiving the reference current, thereference thin film transistor having a gate connected to a gate of thedrive transistor, the reference thin film transistor having a referencenode coupled to a biasing potential for balancing the current mirror,wherein a first node of the drive thin film transistor corresponding tothe reference node is not connected to the biasing potential, therebypreventing a mismatch between the reference thin film transistor and thedrive thin film transistor over time; and a capacitor for storing aprogram voltage independent of a transistor threshold voltage, theprogram voltage settling on the capacitor while the reference current isconveyed through the reference thin film transistor, the capacitorhaving a terminal coupled to both the gate of the drive thin filmtransistor and the gate of the reference thin film transistor.
 2. Thepixel drive circuit according to claim 1, wherein the light emittingdevice is a light emitting diode, and wherein the light emitting diodeincludes a first and a second terminal, the first terminal beingconnected to the first node of the drive transistor and the secondterminal being connected to a first potential.
 3. The pixel drivecircuit according to claim 1, wherein the switching circuitry comprises:a feedback transistor having a gate connected to a first select line, afirst node connected to a data line and a second node connected to thereference transistor; and a switch transistor having a gate connected toa second select line, a first node connected to the data line and asecond node connected to the gate of the reference transistor.
 4. Thepixel drive circuit according to claim 1, wherein the switchingcircuitry comprises: a switch transistor having a gate connected to afirst select line, a first node connected to the data line and a secondnode connected to the gate of the reference transistor; and a feedbacktransistor having a gate connected to a second select line, a first nodeconnected to the gate of the reference transistor and a second nodeconnected to the reference transistor.
 5. The pixel drive circuitaccording to claim 1, wherein the switching circuitry comprises: aswitch transistor having a gate connected to a first select line, afirst node connected to a data line and a second node connected to thereference transistor; a feedback transistor having a gate connected to asecond select line, a first node connected to the reference transistorand a second node connected to the gate of the reference transistor. 6.The pixel drive circuit according to claim 1, wherein the thin filmtransistors are amorphous silicon.
 7. The pixel drive circuit accordingto claim 1, wherein the thin film transistors are polycrystallinesilicon.
 8. The pixel drive circuit according to claim 7 wherein thepolycrystalline silicon is p-type.
 9. The pixel drive circuit accordingto claim 1 wherein the thin film transistors are organic.
 10. The pixeldrive circuit according to claim 9 wherein the organic thin filmtransistors are p-type.
 11. The pixel drive circuit according to claim1, wherein the biasing potential is sufficient to maintain the referencetransistor in a saturation mode while the driving current is conveyed tothe light emitting device and the switching circuitry is deactivated.12. The pixel drive circuit according to claim 1, wherein the biasingpotential is connected to the node of the reference transistor via aswitch transistor controlled separately from the switching circuitry,the switch transistor biasing the reference transistor with the biasingpotential while the switching circuitry is deactivated.
 13. The pixeldrive circuit according to claim 1, wherein the biasing potential isdifferent from a potential at a drain terminal or a source terminal ofthe drive thin film transistor.
 14. A pixel drive circuit for use in adisplay, the pixel drive circuit comprising: switching circuitry forproviding a reference current input; a current mirror for providing adrive current to a light emitting device based on the reference current,the current mirror including: a drive thin film transistor for conveyingthe drive current through the light emitting device, the drive thin filmtransistor being connected in series with the light emitting device viaone of a drain terminal or source terminal of the drive thin filmtransistor; a reference thin film transistor for receiving the referencecurrent, the reference thin film transistor having a gate connected to agate of the drive transistor, the reference thin film transistor havinga node coupled to a biasing potential via one of a source terminal or adrain terminal of the reference thin film transistor, thereby preventinga mismatch between the reference thin film transistor and the drive thinfilm transistor over time, wherein one of a source terminal or a drainterminal of the drive thin film transistor corresponding to the one ofthe source terminal or the drain terminal of the reference thin filmtransistor is not connected to the biasing potential; and a capacitorfor storing a program voltage independent of a transistor thresholdvoltage, the program voltage settling on the capacitor while thereference current is conveyed through the reference thin filmtransistor, the capacitor having a terminal coupled to both the gate ofthe drive thin film transistor and the gate of the reference thin filmtransistor.
 15. The pixel drive circuit according to claim 14, whereinthe light emitting device is connected to the source terminal of thedrive thin film transistor and the biasing potential is connected to thesource terminal of the reference thin film transistor.
 16. The pixeldrive circuit according to claim 1, wherein the reference nodecorresponds to a source terminal of the reference thin film transistorand the first node of the drive thin film transistor corresponds to asource terminal thereof, or wherein the reference node corresponds to adrain terminal of the reference thin film transistor and the first nodeof the drive thin film transistor corresponds to a drain terminalthereof.
 17. The pixel drive circuit according to claim 14, wherein theone of the source terminal or the drain terminal of the reference thinfilm transistor is the source terminal of the reference thin filmtransistor, and wherein the one of the source terminal or the drainterminal of the drive thin film transistor is the source terminal of thedrive thin film transistor.
 18. The pixel drive circuit according toclaim 14, wherein the one of the source terminal or the drain terminalof the reference thin film transistor is the drain terminal of thereference thin film transistor, and wherein the one of the sourceterminal or the drain terminal of the drive thin film transistor is thedrain terminal of the drive thin film transistor.